[sidebar view] Show list item separator lines

Showing lines is the default setting, so not doing so makes the sidebar list visually inconsistent with the various other lists inside QML-based KCMs.

My sense is that the reason why we hid the lines in the past was because the lines had a very heavy appearance, but that was improved recently to look much more lightweight.

Therefore, let's turn the lines on. If we still think the lines are too heavy-looking, we should continue to improve that in Kirigami itself rather than turning them off in random places.

cc @teams/vdg